A technique has been developed for efficient use of heat in flue gas from a thermal power generation boiler. In the technique, heat in boiler flue gas is recovered by a heat recovery unit of a gas-gas heater (GGH) and flue gas to be discharged externally from a stack is heated by a reheater of the GGH. The heat recovery unit is installed on the downstream side of an air preheater in an air pollution control facility for the thermal power generation boiler. The heat recovery unit reduces a temperature of flue gas from a temperature of around 140° C. to around 95° C., for example, to maintain a soot and dust collection efficiency of a low-temperature electronic precipitator. After soot and dust are collected by the electronic precipitator, the flue gas is introduced into a desulfurization device that removes sulfur oxide in the flue gas, and the flue gas is then discharged externally from the stack. The flue gas to be externally discharged is heated by the reheater installed on the downstream side of the desulfurization device using heat recovered by the heat recovery unit to prevent white smoke from being discharged from a stack outlet, and the inside of a flue gas duct from being corroded (refer to Patent Literature 1). In controlling the heat recovery unit and the reheater, a flue gas temperature is measured at an outlet of the heat recovery unit so as to increase soot and dust collection performance of the electronic precipitator and a heating medium temperature is measured at an outlet of the reheater so as to prevent a heat transfer tube of the reheater from being corroded, for example.
An operation control system has been developed for the case where ammonia denitration using ammonia is performed for removing nitrogen oxide in flue gas. The operation control system includes a detection means that detects states of operation parameters relating to production of ammonium chloride contained in flue gas flowing in a heat recovery unit; an analysis device analyzes data detected by the detection means and detects a state of the heat recovery unit to perform operation control (refer to Patent Literature 2).